Merge "More documentation for handling RippleDrawable mask at run time" into mnc-dev

 * attribute identifier.

 * <p>

 * A touch feedback drawable may contain multiple child layers, including a

 * special mask layer that is not drawn to the screen. A single layer may be set

 * as the mask by specifying its android:id value as {@link android.R.id#mask}.

 * special mask layer that is not drawn to the screen. A single layer may be

 * set as the mask from XML by specifying its {@code android:id} value as

 * {@link android.R.id#mask}. At run time, a single layer may be set as the

 * mask using {@code setId(..., android.R.id.mask)} or an existing mask layer

 * may be replaced using {@code setDrawableByLayerId(android.R.id.mask, ...)}.

 * <pre>

 * <code>&lt!-- A red ripple masked against an opaque rectangle. --/>

 * &ltripple android:color="#ffff0000">